It has long been realized that the force commutated cycloconverter is best suited for supplying variable frequency AC power to control the speed of AC machines. That it can be advantageously used for other applications in view of its unique properties, has also been established.
A cycloconverter is comprised of a multiplicity of electronic "switches" (e.g., transistors, thyristors) used to fabricate an AC output voltage waveform from a multiphase input source. This is accomplished by sequentially switching segments of the input voltage waves to the output, so that the desired output waveform is produced. The operating principle of a cycloconverter is well-known in the art and is described in a book entitled "Thyristor Phase-Controlled Converters And Cycloconverters" by B. R. Pelly, published in 1971 by John Wiley and Sons. For the force commutated cycloconverter the output waveform is fabricated in such a manner that the "switches" have the ability to interrupt the flow of current at any time independently of the instantaneous input source voltages and of the load current.
In practice, the switches may be realized either by devices having intrinsic turn-off ability (i.e., transistors or gate controlled switches), or SCR devices often called thyristors with the additional force commutating circuitry which is necessary to terminate the conduction of this latter type of switches. Unfortunately, adequately rated "turn-off" devices which are workable in high power systems are not presently available, therefore the second type of switches, e.g. thyristors, must generally be used.
In the last decade considerable effort has been expended to devise techniques for force commutating thyristors in cycloconverters. However, most of the circuits developed to date have serious shortcomings, and are suitable only for rather limited applications. In view of the great potential advantage of the forced commutated cycloconverter, it is desirable to improve on the present commutating circuits, especially for the commutation of the "Unrestricted Frequency Changer" (U.F.C.) with three-phase output, since this system has been shown to be most practical and economical in industrial applications. (For a broad definition of the Unrestricted Frequency Changer see in "Electronics Engineers Handbook", First Edition, 1975, McGraw-Hill Co., Section 15-42, page 15-52 under "Power Frequency Changers" by L. Gyugyi. A typical unrestricted frequency changer has been described in U.S. Pat. No. 3,170,107 of R. D. Jessee.)
There are basically three approaches to force commutation of the main thyristors of a cycloconverter:
1. Individual switch commutation (i.e., commutating each conducting switch separately), PA0 2. Input line commutation (i.e., commutating all switches at the input lines), PA0 3. Load commutation (i.e., commutating all switches at the load).
These approaches have relative advantages and disadvantages.
1. Individual Switch Commutation
Individual switch commutation provides for separate commutation of each switching element in the cycloconverter. Separate commutation of individual switches may be accomplished by two basic arrangements (as shown in FIG. 1 described hereinafter). The first arrangement consists in using for each switch two thyristors connected in antiparallel in the power path and in providing a commutating circuit in parallel thereto. The second arrangement (as shown in FIG. 2 described hereinafter) consists in having the power thyristor to be commutated mounted in the central branch of a rectifier bridge connected in the power path, and providing a commutating circuit in parallel thereto. A force commutated cycloconverter which is typical of a first mode of individual switch commutation applied to cycloconverters is disclosed in U.S. Pat. No. 3,302,093 of C. J. Yarrow issued Jan. 31, 1967. This circuit combines natural and force commutation depending on the voltage direction between the thyristor to be turned off. Force commutation is achieved in the Yarrow patent by adding only passive components to the basic cycloconverter. The operation is automatic, force commutation occurring at each required instant as a result of firing the next pair of anti-parallel thyristors in sequence. This is an advantage. However, the voltage charging the commutating capacitors is determined by the difference between two instantaneous input voltages at the moment of firing. This difference may be small when the load current is large. Consequently, the commutating capability of the Yarrow circuit varies over the output cycle and it depends upon the load. This requires restricting the operating conditions in order to be able to maintain commutating ability. In a second known form of individual force commutation (as illustrated in FIG. 4 described hereinafter) firing pulses are applied to thyristors of a positive bank of thyristors when the load is positive, to the thyristors of a negative bank of thyristors when the load is negative, when the conditions are right so that natural commutation occurs; otherwise, force commutation is effected with commutating capacitors which are charged to some voltage prior to the instant of commutation. In contrast to the first form of individual force commutation the capacitors here do not follow the instantaneous voltage difference between phases but keep their maximum voltage. Therefore, the commutation capability of the second form of individual force commutation just mentioned is superior to the one of the Yarrow circuit previously mentioned. However, much higher ratings are required for the thyristors. Also, the commutating ability is still dependent upon operating conditions of the cycloconverter, which for this reason must be restricted accordingly.
A third form of individual force commutation applied to cycloconverters has been described by L. J. Ward and W. Sinclair in a paper entitled "Production of Constant Frequency Electrical Power for Aircraft Using Static Equipment," presented in 1962 at a joint conference in England of the Royal Aeronautical Society and the Institution of Electrical Engineers. (This circuit is illustrated in FIG. 5 hereinafter described). In such case, the main switching thyristors of the cycloconverter must always be force commutated. A capacitor is charged during conduction of one thyristor, discharged when the other is fired to turn off the first one and charged to a reverse polarity to turn off the second one. When the first thyristor is again fired, the capacitor is discharged and brought back to the original charge and polarity, ready for the next force commutation. With this arrangement the number of main cycloconverter thyristors and commutating capacitors is halved by comparison to the previous form of individual commutation. However, this circuit is also load sensitive. There is also the inconvenience of reversing the voltage of the commutating capacitor for the next commutation, since it entails two operations of the discharge circuit for one conduction interval. This increases the losses and the rating of the thyristor.
More generally, the above-mentioned three circuits of the prior art have in common the disadvantage that their commutation capability is load dependent. Therefore, for a reliable operation under all conditions, additional components are required.
2. Input Line Commutation
With this type of commutation a commutating circuit is connected, as illustrated in FIGS. 6 and 7 described hereinafter between the phase lines of the power source, for a cycloconverter in a Bridge or Wye configuration. To commutate, it is necessary that the commutation circuit decrease the input line voltage to the thyristor to be turned off below the line voltage to the thyristor to be fired.
At each commutation instant the voltage of the input line feeding the conducting "outgoing" thyristor must be forced to drop below that of the line feeding the "oncoming" thyristor. If one considers a three phase cycloconverter it is clear that each output phase should be treated as a separate single-phase unit and that the inputs to each unit be isolated from the others to prevent undesirable interaction of the commutation circuits. In general, such operation is not desired. Input line commutation requires that either separate commutating circuits be provided for each input line, or that several common commutating circuits be used and connected appropriately to each input line by "steering" thyristors.
3. Load Commutation
One typical known mode of load commutation with cycloconverters provides full commutating capability under all operating conditions. A single commutating circuit achieves commutation of all main thyristors for one phase, thus giving a good utilization of commutating components.
It appears from the preceding enumeration of the three basic arrangements for a forced commutation cycloconverter that in both individual and input line commutation, the commutation pulse is applied at a separate point for each main thyristor. This requires many individual commutating circuits. Also for a reliable operation, independent from load current and instantaneous voltage levels, each individual commutating circuit would have to include many components, the total number of which would be excessive.
With load commutation however, the commutation pulse is applied at a point common to a number of main thyristors, and the number of separate commutation circuits required is therefore reduced. With the number of commutating circuits and thyristors reduced the basic control can also be simpler. The present invention makes generally use of this third approach to commutation, and therefore affords the advantages derived therefrom. However, this particular approach does not exclude the use of the input line commutation approach whenever the cycloconverter is used in a condition of reverse flow of power for a particular application, for instance in an aircraft electrical plant which can be used both in flight with the engine as the source of power and on the ground with a local source of power for the purpose of starting the engine, the alternator coupled to the engine being the load in the later instance.
One object of the present invention is to provide an unrestricted static frequency changer system of improved capability through the use of forced commutation.
Another object of the present invention is to provide an improved force commutated cycloconverter of the type using load commutation.
Still another object of the present invention is to provide a cycloconverter of the force commutation type using directly a commutating capacitor in conjunction with the switches of the cycloconverter.
A further object of the present invention is to apply direct capacitor force commutation to a cycloconverter.
One object of the invention is to provide force commutation in cycloconverter which is independent from load conditions.
An additional object of the present invention is to provide a cycloconverter of the force commutation type which is free from the usual restrictions of operation for commutating capability.